The religion of the Proto-Indo-Europeans is not directly attested, but reconstruction has been attempted based on the existence of similarities among the deities, religious practices and mythologies of the Indo-European peoples. The hypothesized reconstructions below are based on linguistic evidence using the comparative method. Archaeological evidence is difficult to match to any specific culture in the period of early Indo-European culture in the Chalcolithic.[1][full citation needed] Other approaches to Indo-European mythology are possible, such as the trifunctional hypothesis of Georges Dumézil.[2]

Linguists are able to reconstruct the names of some deities in the Proto-Indo-European language (PIE) from many types of sources. Some of the proposed deity names are more readily accepted among scholars than others.[3]

The form Arap Ushas for *H2eus(os) appears in Albanian folklore, but is a name of the Moon. See also the names for the Sun which follow. An extension of the name may have been *H2eust(e)ro,[13] but see also the form *as-t-r, with intrusive -t- [between s and r] in northern dialects".[14] Anatolian dialects: Estan, Istanus, Istara; Greek, Hestia, goddess of the hearth; Latin Vesta, goddess of the hearth; in Armenian as Astghik, a star goddess; possibly also in Germanic mythology as Ēostre or Ostara; and Baltic, Austija. The name *Deh2nu- has been connected with the Dan rivers which run into the Black Sea (Dnieper, Dniester, Don, and Danube) and other river names in Celtic areas.

The Sun and Moon are often seen as the twin children of various deities, but in fact the sun and moon were deified several times and are often found in competing forms within the same language. The usual scheme is that one of these celestial deities is male and the other female, though the exact gender of the Sun or Moon tends to vary among subsequent Indo-European mythologies. The original Indo-European solar deity appears to have been female,[15] a characteristic not only supported by the higher number of sun goddesses in subsequent derivations (feminine Sól, Saule, Sulis, Solntse—not directly attested as a goddess, but feminine in gender—Étaín, Grían, Aimend, Áine and Catha versus masculine Helios, Surya, Savitr, Usil and Sol; Hvare-khshaeta is of neutral gender), but also by vestiges in mythologies with male solar deities (Usil in Etruscan art is depicted occasionally as a goddess, while solar characteristics in Athena and Helen of Troy still remain in Greek mythology). The original Indo-European lunar deity appears to have been masculine,[15] with feminine lunar deities like Selene, Minerva and Luna being a development exclusive to the eastern Mediterranean. Even in these traditions, remnants of male lunar deities, like Menelaus, remain.

Analysis of different Indo-European tales indicate the Proto-Indo-Europeans believed there were two progenitors of mankind: *Manu- ("Man"; Indic Manu; Germanic Mannus) and *Yemo- ("Twin"; Indic Yama; Germanic Ymir), his twin brother. Cognates of this set of twins appear as the first mortals, or the first gods to die, sometimes becoming the ancestors of everyone and/or king(s) of the dead.[16][17]

Horse Twins, usually have a name that means 'horse' *ekwa-, but the names are not always cognate, because there is no lexical set (Mallory & Adams 2006, p. 432). They are always male and usually have a horse form, or sometimes, one is a horse and the other is a boy. They are brothers of the Sun Maiden or Dawn goddess, sons of the sky god, continued in Sanskrit Ashvins and Lithuanian Ašvieniai, identical to Latvian Dieva deli. Other horse twins are: Greek, Dioskouroi (Polydeukes and Kastor); borrowed into Latin as Castor and Pollux; Irish, the twins of Macha; Old English, Hengist and Horsa (both words mean 'stallion'), and possibly Old Norse Sleipnir, the eight-legged horse born of Loki; Slavic Lel and Polel; possibly Christianized in Albanian as Sts. Flori and Lori. The horse twins may be based on the morning and evening star (the planet Venus) and they often have stories about them in which they "accompany" the Sun goddess, because of the close orbit of the planet Venus to the sun (JIES 10, 1&2, pp. 137–166, Michael Shapiro, who references D. Ward, The Divine Twins, Folklore Studies, No. 19, Univ. Calif. Press, Berkeley, 1968).

A fuller treatment of the subject of the Indo-European Pantheon would not merely list the cognate names but describe additional correspondences in the "family relationships", festival dates, associated myths (but see Mythology section) and special powers.

"Pandemonium" is Jaan Puhvel's word for the mutual demonization that occurred when the Younger-Avesta demonized the daevas, and the post-Rigvedic texts demonized the asuras. Neither demonization occurs in the oldest texts: in the Rigveda, there is not yet any hard-and-fast distinction between asuras and dēvas, and even in the later Vedas, the two groups (though thematically in opposition) cooperate at certain times.[18] In the Old Avestan texts the daevas are to be rejected for being misguided by the "lie", but they are still gods, and not demons.[19]

However, in the 19th century this distinction between the older and younger texts had not yet been made, and in 1884 Martin Haug "postulated his thesis that the transition of both the words [asuras and devas] into the designations of the demons ... is based on a prehistoric schism in religion ..."[20] The observation was reiterated by Jacob Grimm (DM3, p. 985), who, like Haug, considered it to be the theological basis of Zoroastrianism's dualism.

Before this (in the 1850s), Westergaard had attributed the Younger-Avesta's demonization of the daevas to a "moral reaction against Vedic polytheism", but that (unlike the general notion of a mutual demonization) was very quickly rejected, and by 1895 James Darmesteter noted that it has "no longer [had] any supporter."[21] Nonetheless, some modern authors like Mallory and Adams still refer to Zoroastrianism as a "religious reformation" of Vedic religion (Mallory & Adams 2006, pp. 408–09). Most scholars however stress that there were two independent developments in ancient Iran and post-Rigvedic India, but nonetheless to be considered against the common background of prehistoric Indo-Iranian religion where both groups coexisted, with the asuras, perhaps even as a subset (having a particular common characteristic, like the Adityas) of the daevas, the national gods.

Related to the dragon-slaying myth is the "Sun in the rock" myth, of a heroic warrior deity splitting a rock where the Sun or Dawn was imprisoned. Such a myth is preserved in the Rigvedic story of Vala, where Ushas and the cows, stolen by the Panis were imprisoned, connected with other myths of abductions into the netherworld such as the mysteries of Eleusis connected with Persephone, Dionysus and Triptolemus.

The Sun god, Helios of Greek mythology, Surya of Hinduism, and Sól of Germanic mythology are represented as riding in a chariot with horses. (Note that the chariot was not invented until Indo-European had already split into linguistic branches[citation needed].)

The analysis of different Indo-European tales indicates that the Proto-Indo-Europeans believed there were two progenitors of mankind: *Manu- ("Man") and *Yemo- ("Twin"), his twin brother. A reconstructed creation myth involving the two is given by David W. Anthony, attributed in part to Bruce Lincoln:[23] Manu and Yemo traverse the cosmos, accompanied by the primordial cow, and finally decide to create the world. To do so, Manu sacrifices either Yemo or the cow, and with help from the sky father, the storm god and the divine twins, forges the earth from the remains. Manu thus becomes the first priest and establishes the practice of sacrifice. The sky gods then present cattle to the third man, *Trito, who loses it to the three-headed serpent *Ngwhi, but eventually overcomes this monster either alone or aided by the sky father. Trito is now the first warrior and ensures that the cycle of mutual giving between gods and humans may continue.

There are almost no mythological tales from Rome, but the early "history" of Rome is recognized as being an historicized version of various old myths. Romulus and Remus were twin brothers. They both have stories in which they are killed.

Remus is killed by his brother Romulus at the foundation of Rome;

Romulus is dismembered by the senators, "[…] there were some who secretly hinted that he had been torn limb from limb by the senators […]" There is no world making here, but Romulus is the eponymous ancestor of the Romans, and the founder of Rome.[24]

Gemini is the actual Latin word for 'twins' though it usually applies to Castor and Pollux, see Horse Twins in the Pantheon section. They were worshipped all over the Roman world with votive altars with inscriptions, which remained after the Romans were gone. This may be the source of some names which appear in early Christian myths.[clarification needed]

The Germanic languages have information about both Ymir and Mannus (cognates of *Yemo- and *Manu- respectively), but they never appear in the same myth, rather they appear only in myths widely separated in both time and circumstances.

A Roman text (dated CE 98) tells that Mannus, the son of Tuisto, was the ancestor of the Germanic people, according to Tacitus, writing in Latin, in Germania 2. We never see this being again, but the name Allemagne is interpreted (perhaps by folk etymology) as "all-men" the name for themselves.

Celtic (in this case Irish) texts were written down between the 11th and 14th centuries CE. In one myth a bull is killed and dismembered by another bull and the parts of his body are distributed around Ireland, which explains the names of many features of the landscape, though not the cause of their existence. "It was not long before the men of Erin [Ireland], as they were there in the company of Ailill and Madb early on the morrow, saw coming over Cruachan from the west, the Brown Bull of Cualnge with the Whitehorned [Bull] of Ai in torn fragments hanging about his ears and horns." An example of one of the distributions is this one: "Then he raised his head, and the shoulder-blades of the Whitehorned fell from him in that place. Hence, Sruthair Finnlethe ('Stream of the White Shoulder-blade') is the name given to it."[25]

In Zoroastrianism and Persian mythology, according to the Zoroastrian story of creation, Ahura Mazda existed in light in goodness above, while Angra Mainyu existed in darkness and ignorance below. They have existed independently of each other for all time, and manifest contrary substances. Ahura Mazda first created seven abstract heavenly beings called Amesha Spentas, who support him and represent beneficent aspects, along with numerous yazads, lesser beings worthy of worship. He then created the universe itself in order to ensnare evil. Ahura Mazda created the floating, egg-shaped universe in two parts: first the spiritual (menog) and 3,000 years later, the physical (getig). Ahura Mazda then created Gayomard, the archetypical perfect man, and the first bull.[26] While Ahura Mazda created the universe and humankind, Angra Mainyu, whose instinct is to destroy, miscreated demons, evil yazads, and noxious creatures (khrafstar) such as snakes, ants, and flies. Angra Mainyu created an opposite, evil being for each good being, except for humans, which he found he could not match. Angra Mainyu invaded the universe through the base of the sky, inflicting Gayomard and the bull with suffering and death. However, the evil forces were trapped in the universe and could not retreat. The dying primordial man and bull emitted seeds. From the bull's seed grew all beneficial plants and animals of the world, and from the man's seed grew a plant whose leaves became the first human couple. Humans thus struggle in a two-fold universe trapped with evil. The evils of this physical world are not products of an inherent weakness, but are the fault of Angra Mainyu's assault on creation. This assault turned the perfectly flat, peaceful, and ever day-lit world into a mountainous, violent place that is half night.[26]

In Lithuanian, a folktale tells of a bull and three cows which are beheaded by Aušrinė, (the morning star) and then the land appears. "The maiden upon returning released her bull. The bull knelt down and spoke in a man's voice: "Chop off my head!" The maiden did not want to chop it off, but she had to. She chopped the head off—a fourth of the seas disappeared, became land. Her brother emerged from the bull. She cut off the heads of all three cows, who were her sisters. All the seas disappeared, turned to land. The earth sprang to life."[27]

Culture myths, stories in which some godlike being teaches the "arts of civilization" (actually technologies) to humans, are found in all cultures. The culture myths of the Indo-Europeans tell how the culture gods taught humans such arts as how to make fire, the proper way to kill and butcher an animal (sacrifice), religious rituals and law codes, smithing, weaving, ploughing, and healing. Culture-giving figures (e.g. Prometheus and Loki) sometimes have an intermediate position between gods and humans (i.e., demigods). They are certainly supernatural, but they often die or are tortured by other gods for their beneficence to humans; nevertheless they are often revived and worshipped like regular gods or revered as heroes. Mallory and Adams call them Craft Gods and argue that they are not linguistically reconstructible; however, Cox compares Greek Prometheus with Hindu Pramanthu (Cox, p. 421). Smith gods, a subset of the culture gods, are slightly reconstructible according to Mallory and Adams.

Flood myth is a widespread myth, in Indo-European as in other cultures (for example, Noah and Utnapishtim myths). In Hindu mythology, there is a story of a great flood, wherein the Matsya Avatar of Vishnu warns the first man, Manu. In Greek mythology, Deucalion is the survivor of the flood, and his sons Aeolus, Ion and Dorus are the founders of the three main lines of modern Greeks.

Émile Benveniste states that "there is no common [IE] term to designate religion itself, or cult, or the priest, not even one of the personal gods".[28] There are, however, terms denoting ritual practice reconstructed in Indo-Iranian religion which have root cognates in other branches, hinting at common PIE concepts. Thus, the stem *hrta-, usually translated as "(cosmic) order" (Vedic ṛta and Iranian arta[29]). Benveniste states, "We have here one of the cardinal notions of the legal world of the Indo-Europeans to say nothing of their religious and moral ideas" (pp. 379–381). He also adds that an abstract suffix -tu formed the Vedic stem ṛtu-, Avestan ratu- which designated order, particularly in the seasons and periods of time. The same root and suffix, but a different formation, appears in Latin rītus "rite".

The following list of reconstructed PIE religious terms is based on EIEC[30] and Lyle Campbell[31]

Benveniste also posits the existence of a dual conception of sacredness, divided into a positive side, the intrinsic, otherworldly power of deities; and a negative side, sacredness of objects in the world that make them taboo for humans. This opposition is found in word pairs such as the Latin sacer/sanctus and Greek ἅγιος/ἱερός.[33]

^In order to present a consistent notation, the reconstructed forms used here are cited from Mallory & Adams 2006. For further explanation of the laryngeals – <h1>, <h2>, and <h3> – see the Laryngeal theory article.

^One of the original sources for the stories of Romulus and Remus is Livy's History of Rome, vol. 1, parts iv–vii and xvi. This has been published in an Everyman edition, translated by W. M. Roberts, E. P. Dutton & Co., New York 1912.

^The original source is the last chapter of the Táin Bó Cúalnge, usually called in English, The Cattle Raid of Cooley. These quotations are from The Ancient Irish Epic Tale, Táin Bó Cúalnge, translated by Joseph Dunn, publ. David Nutt, London, 1914.

^The original source for this is a folktale called Saulė and Vejų Motina (The Sun and the Mother of the Winds), pp. 309–13, of M. Davainis-Silvestraitis' Collection, Pasakos, Sakmės, Oracijos (Tales, Legends and Orations) published in Vilnius, 1973. The English version is from p. 67 Of Gods and Men by Algirdas J. Greimas, translated by Milda Newman, Indiana Univ. Press, Indianapolis, 1992.